1. Field of the Invention
This invention relates to a distributed feedback semiconductor laser device and a distributed Bragg reflection semiconductor laser device which attains laser oscillation at a wavelength ranging from 660 to 890 nm in a single longitudinal mode.
2. Description of the Prior Art
Semiconductor laser devices, which are used as light sources for optical information processing systems and optical measurement systems utilizing optical fibers therein, are required to have operation characteristics achieving laser oscillation in a single longitudinal mode. Laser devices are known which achieve laser oscillation in a single longitudinal mode, such as distributed feedback semiconductor lasers and distributed Bragg reflection semiconductor lasers which have a diffraction grating with a periodic corrugation on or near the active region.
FIG. 3 shows a typical conventional distributed feedback semiconductor laser, which comprises an n-InP substrate 10, an n-InP cladding layer (buffer layer) 20, a non-doped InGaPAs active layer 30, a p-InGaPAs optical guiding layer 40, a p-InP cladding layer 50 and a p-InGaPAs cap layer 60, in sequence. A p-side ohmic electrode 70 and an n-side ohmic electrode 80 are disposed on the cap layer 60 and the substrate 10, respectively. A diffraction grating 401 for laser oscillation is formed on the upper surface of the optical guiding layer 40. This laser device utilizes the InGaPAs-InP system which achieves laser oscillation at a wavelength of as long as 1,300 nm.
On the other hand, a semiconductor laser device attaining laser oscillation at a shorter wavelength of 890 nm or less comprises, as shown in FIG. 4, an n-GaAs substrate 100, an n-GaAlAs cladding layer 200, a non-doped GaAs or GaAlAs active layer 300, a p-GaAlAs optical guiding layer 400, a p-GaAlAs cladding layer 500 and a p-GaAs cap layer 600, in sequence. However, this laser has the GaAlAs cladding layer 500 on a diffraction grating 401 which is formed on the GaAlAs optical guiding layer 400, so that the GaAlAs alloy crystal is readily oxidized in the atmoshere in the formation process of the diffraction grating 401 to thereby from an oxide film on the GaAlAs optical guiding layer 400 which makes it difficult to grow the succeeding crystal (i.e, the GaAlAs cladding layer 500) thereon. This, it can be said that semiconductor lasers having a diffraction grating therein for laser oscillation at a wavelength of as short as 890 nm or less have not yet been sufficiently developed.